1. Field of the Invention
The present invention relates to a digital camera for capturing a subject image to generate image data, and especially to an autofocus (hereinafter referred to as xe2x80x9cAFxe2x80x9d) technique used in a digital camera.
2. Description of the Background Art
Most of single lens reflex digital cameras are configured such that an image sensor such as a CCD is installed in a film loading part of a single lens reflex camera utilizing silver halide films (hereinafter referred to as a xe2x80x9csilver-halide-film cameraxe2x80x9d). Such digital cameras achieve focus by a phase difference autofocus (AF) method in which focusing is obtained by separating light passing through a taking lens by a separator lens and obtaining the amount of lens travel on the basis of a distance between images.
The recent trend to increase the number of pixels with reduced pixel pitch in an image sensor requires improved focus accuracy. In the phase difference AF method, however, there are limitations to the focus accuracy, and therefore it is getting difficult to perform autofocusing with focus accuracy appropriate to the pixel pitch.
Besides the phase difference AF method, AF techniques include image-signal autofocus that attains focus on the basis of contrast of image data obtained in an image sensor: more specifically, a contrast AF method for achieving focus based on image contrast. The contrast AF method attains higher focus accuracy than the phase difference AF method. Thus, it is desired that a digital camera should adopt the contrast AF method depending on an increased number of pixels in the image sensor. However, as above described, contrast AF cannot be used in a single lens reflex digital camera.
Now, if a digital camera is configured with a CCD image sensor which includes a plurality of pixels more densely packed than before, a permissible circle of confusion becomes smaller than before and a higher degree of focus-position detection accuracy is required in autofocusing.
Conventionally, a technique, called a xe2x80x9ccontrast AF methodxe2x80x9d (or xe2x80x9chill-climbing AF methodxe2x80x9d) is adopted for autofocusing in image capture devices such as video cameras. This contrast AF method is more specifically a technique which is used while driving a focusing lens included in a taking lens, and is for obtaining as evaluation values the contrast of captured images at each drive step and selecting a lens position with the maximum evaluation value as an in-focus position. Hereinafter, the terminology xe2x80x9cin-focus positionxe2x80x9d refers to a position at which a lens is positioned to provide an in-focus image.
In the field of video cameras and the like for motion image capture, since the number of pixels in the CCD image sensor to be used is approximately several hundreds of thousands, a permissible circle of confusion is large and improved accuracy is not required in autofocusing. Further, an excessively high focus speed in video recording causes frequent changes of a focused portion of an image in response to movements of the camera and the subject and thereby unnatural images are produced because the human eye cannot follow such frequency. From this, required features of video cameras for autofocusing differ from those of still cameras.
Digital cameras for still image capture, on the other hand, are required to immediately achieve focus in order not to lose a shutter release opportunity.
Besides, the phase difference AF method is conventionally adopted for autofocusing in single lens reflex cameras for silver halide films. In autofocusing by the phase difference AF method, the extent to which an in-focus position of a lens is offset from a film plane can be recognized instantaneously by the distance (phase difference) between images at the time when a phase difference detection sensor with a CCD line sensor receives light from a subject image. From this, the phase difference AF method is advantageous in that only one lens drive brings the in-focus position into coincidence with the film plane.
On the other hand, in autofocusing by the contrast AF method especially when the contrast of a captured image is low, a change: in the evaluation values before and after a taking lens drive may be so small that in which direction the lens should be moved to its in-focus position cannot be determined. The contrast AF method therefore has a problem of taking much time to achieve an in-focus condition.
Even in autofocusing by the phase difference AF method, it is necessary to improve the resolution of the CCD line sensor to achieve an in-focus condition with high accuracy. This increases the size and cost of the phase difference detection sensor for detecting the phase difference. Moreover, the phase difference AF method may have an error in the in-focus position because of an error in installation of the phase difference detection sensor.
The present invention is directed to a digital camera.
According to an aspect of the present invention, the digital camera comprises an image sensor for capturing a subject image; a mirror movable between a first position to enter an optical path from an imaging optical system to the image sensor, and a second position to withdraw from the optical path; a driver for driving the mirror; a first detector for detecting an in-focus condition of the imaging optical system on the basis of an image signal from the image sensor; a display for displaying an image signal obtained by the image sensor; and a focus controller for when the display provides a display, controlling the driver to move the mirror to the second position and driving the imaging optical system according to a result of detection by the first detector.
Thus, when the display provides a display, autofocusing based on the image signal from the image sensor can be performed. Such autofocusing for the live view display provides precise framing.
According to another aspect of the present invention, the digital camera comprises: an image sensor for capturing a subject image; a mirror movable between a first position to enter an optical path from an imaging optical system to the image sensor, and a second position to withdraw from the optical path; a driver for driving the mirror; a first detector for detecting an in-focus condition of the imaging optical system on the basis of an image signal from the image sensor; a second detector for detecting an in-focus condition of the imaging optical system by a phase difference method; a selector for selecting either the first detector or the second detector; and a driver controller for when the selector selects the first detector to detect an in-focus condition, controlling the driver to move the mirror to the second position and when the selector selects the second detector to detect an in-focus condition, placing the mirror at the first position.
The selection from the first and second detectors allows switching between the live view display and the optical viewfinder. Thereby autofocusing can be performed even for framing by the optical viewfinder.
According to still another aspect of the present invention, the digital camera comprises: an image sensor for capturing a subject image; a mirror movable between a first position to enter an optical path from an imaging optical system to the image sensor, and a second position to withdraw from the optical path; a driver for driving the mirror; a first detector for detecting an in-focus condition of the imaging optical system on the basis of an image signal from the image sensor; a second detector for detecting an in-focus condition of the imaging optical system by a phase difference method; an operating member for image capture being movable from a first operating position to a second operating position which is a further pressed position from the first operating position; and a focus controller for when the operating member for image capture is at the first operating position, placing the mirror at the first position and driving the imaging optical system according to a result of detection by the second detector and when the operating member for image capture is at the second operating position, controlling the driver to move the mirror to the second position and driving the imaging optical system according to a result of detection by the first detector.
By driving the imaging optical system according to the result of detection by the second detector, a rough in-focus condition is achieved. Subsequently by driving the imaging optical system according to the result of detection by the first detector with the operating member for image capture at the second operating position, a more precise in-focus condition can be achieved with efficiency.
According to still another aspect of the present invention, the digital camera comprises: an image sensor for capturing a subject image; a mirror movable between a first position to enter an optical path from an imaging optical system to the image sensor, and a second position to withdraw from the optical path; a driver for driving the mirror; a first detector for detecting an in-focus condition of the imaging optical system on the basis of an image signal from the image sensor; a second detector for detecting an in-focus condition of the imaging optical system by a phase difference method; and a focus controller for after driving the imaging optical system according to a result of detection by the second detector with the mirror located at the first position, then controlling the driver to move the mirror to the second position and driving the imaging optical system according to a result of detection by the first detector.
By driving the imaging optical system according to the result of detection by the second detector, a rough in-focus condition is achieved. Subsequently by moving the mirror to the second position and driving the imaging optical system according to the result of detection by the first detector, a more precise in-focus condition can be achieved with efficiency.
In this fashion, an object of the present invention is to enable autofocusing responsive to an image signal from an image sensor in a digital camera and to obtain a focus condition of an image formed in the image sensor with great accuracy and efficiency.
These and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention when taken in conjunction with the accompanying drawings.